Multimode fibers are successfully used in high-speed data networks. However, they are affected by intermodal dispersion, which results from the fact that, for a particular wavelength, several optical modes propagate simultaneously along the fiber, carrying the same information, but travelling with different propagation velocities. Modal dispersion is expressed in terms of Differential Mode Delay (DMD), which is a measure of the difference in pulse delay (ps/m) between the fastest and slowest modes traversing the fiber.
In order to minimize modal dispersion, the multimode optical fibers used in data communications generally comprise a core showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “α-profile,” as follows:
            n      ⁡              (        r        )              =                            n          0                ⁢                              1            -                          2              ⁢                                                Δ                  ⁡                                      (                                          r                      a                                        )                                                  α                                                    ⁢                                  ⁢        for        ⁢                                  ⁢        r            ≤      a        ,
where:
n0 is a refractive index on an optical axis of a fiber;
r is a distance from said optical axis;
a is a radius of the core of said fiber;
Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and
α is a non-dimensional parameter, indicative of a shape of the index profile.
When a light signal propagates in such a core having a graded index, the different modes experience a different propagation medium, which affects their speed of propagation differently. By adjusting the value of the parameter α, it is thus possible to theoretically obtain a group velocity, which is virtually equal for all the modes and thus a reduced intermodal dispersion for a particular wavelength. However, an optimum value of the parameter α is valid for a particular wavelength only. Furthermore, the exact parameter value α, as well as the actual shape of the refractive index profile, are difficult to control during manufacture of the fiber.
As a consequence, intermodal dispersion cannot be completely dismissed nor neglected.
In such graded-index multimode fibers, groups of modes with substantially identical propagation constants exist. Hence, the optical modes traveling along the fiber are sorted into mode groups, which are defined such that modes of the same mode group exhibit nearly the same group index. Group index difference between neighboring mode groups, and thus time of flight difference, are nearly the same for all mode groups.
The propagation of Mode Groups through optical components, such as optical fibers, has been subject to investigations for a long time.
High speed multimode optical fibers such as OM4 fibers (which are laser-optimized, high bandwidth 50 μm multimode fibers, standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard, each of which is hereby incorporated by reference in its entirety) reach maximum speed if all Mode Groups experience the same ‘time of flight’ while passing through the fiber. The design of such fibers depends strongly on DMD (Differential Mode Delay) measurements at the operational wavelength (850 nm).
If the different Mode Groups do not experience the same time of flight, the ‘DMD profile’ broadens, and the related optical bandwidth decreases. This means in practice the maximum number of bits per second, which may be transmitted by the optical fiber, is limited to a value determined by the measured bandwidth value. If the DMD profile is slightly wider than the ideal profile (which corresponds to a profile in which all pulses leaving the fiber experience the same time of flight, and show the same shape as the laser pulses launched into the fiber), some Mode Groups are not transmitted ideally by the fiber, and cause a decrease in bandwidth.
In today's state of the art, it is however not possible to know exactly which Mode Groups cause the broadening of the DMD profile.
The same fact may be acknowledged for other optical components like fiber-to-fiber couplers, attenuators and detectors. Although their overall performance and characteristics may be assessed, it is not possible to know how Mode Groups behave while passing through such optical components, nor how they may play a role on their performance.
Patent document U.S. Pat. No. 5,251,022, which is hereby incorporated by reference in its entirety, describes a measurement system, which quickly and nondestructively characterizes the mode-dependent losses and coupling of a multi-mode, graded-index, connectorized, passive fiber optic component.
Such a measurement system allows determining the mode transition matrix of the optical component under test and comprises:                mode selective launcher means having a plurality of optical paths with varying launch conditions;        input-output optic means having at least one reference optical path and at least one optical path for the passive fiber optic component under test;        mode selective detection means having a plurality of optical paths with a variety of mode filters, and        means for data storage and matrix calculation.        
The mode transition matrix, as well as the modal power vector, is calculated using measurements of the optical power propagated through optical courses comprising the optical paths. In other words, the system uses mode filters and fiber optic switches to create optical paths, with the measurement of power propagating through each optical path being used to determine the mode transition matrix of the passive fiber optic component.
Such a measurement system is hence quite complex and cumbersome. Actually, it requires a serial combination of a mode selective launcher, an input/output section, and a mode selective detection section, each section containing a pair of optical switches.
Patent document EP 2 579 483, which is hereby incorporated by reference in its entirety, provides a method and a related apparatus for transmitting optical signals over a multi-mode fiber using spatial multiplexing. Optical signals are multiplexed into different principle mode groups of a graded-index multi-mode fiber. After transmitting the space division multiplexed optical signal over the multi-mode fiber, an optical Fourier transformation is performed, for instance by a lens, to spatially separate the multiplexed optical signals from the space division multiplexed optical signal, as rings with different radii. In other words, at the receiver, different principle mode groups are separated through an optical Fourier transformation.
Hence, patent document EP 2 579 483 provides a method for spatial mode groups separation through optical Fourier transformation. However, it does not allow assessing the individual behavior of mode groups while passing through an optical component.
It would hence be desirable to provide a simple method for characterizing Mode Group properties of multimodal light traveling through optical components, which would give experimental knowledge of how mode groups behave while passing through such optical components, like for instance optical fibers.
Such an experimental knowledge would serve design improvement of optical fibers and would allow, among others, achieving the best possible design for the highest quality of multimode fibers. It would hence allow increasing bit rates in multimodal optical systems.